Cellular positioning system (CPS)

ABSTRACT

Positioning system for locating a mobile body comprising a plurality of earth based spread spectrum (SS) broadcasting stations arranged geographically in a cellular pattern. Each SS broadcasting station include a modulator providing a channel signal structure which is substantially orthogonal with respect to adjacent stations in the cellular pattern, each channel signal including navigation beacon data including a unique beacon identification, station latitude and longitude, time --  slot and phase characterizations and selected parameters of adjacent stations. In one embodiment, each modulator provides a chirped SS signal in which the navigation beacon is a frequency tone that is repeatedly swept over a selected frequency band for each station. In a further embodiment, each modulator provides a GPS like direct sequence SS signal in which the navigation beacon is a PN coded broadcast. A receiver on the mobile body receives the SS signals from at least three of the SS broadcasting stations and determines the location thereof. A fourth SS broadcasting station provides altitude. CPS satellite signals can be used for timing control.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.08/363,773 filed Dec. 23, 1994 now pending entitled "POSITION ENHANCEDCOMMUNICATION SYSTEM INCLUDING SYSTEM FOR EMBEDDING CDMA NAVIGATIONBEACONS UNDER THE COMMUNICATIONS SIGNALS OF A WIRELESS COMMUNICATIONSYSTEM", incorporated herein by reference.

BRIEF DESCRIPTION OF THE INVENTION

Wireless communications are rapidly augmenting conventional telephonecommunications. For many types of wireless calls such as 911 or callsfor roadside automotive repair/towing, knowing and conveying thelocation of the call origin is vital. However, since most users ofwireless communications are mobile, their location is typically notknown and can encompass a large uncertainty region. As shown inapplication Ser. No. 08/363,773, assigned to the assignee hereof, thereare several alternative systems for position location by mobile users,but none of these current systems are adequate for the wide variety ofenvironments for wireless communications. By far the largest segment ofmobile communications in the US is the current cellular voice system,and this system presents an opportunity to establish a cellular array ofspread spectrum navigation beacons that can be used for positiondetermination by users of cellular telephony and other public wirelessservices. This concept was described in the above-referenced patentapplication. The present invention discloses about how a set of spreadspectrum navigation beacons can be uniquely designed and arranged in acellular pattern, and how the required navigation receiver signalprocessing can be efficiently integrated into a cellular phone via novelapplication of state-of-the-art technology. As shown later herein, thecellular array of navigation beacons can be a stand-alone navigationsystem, or it can be co-located and integrated with an existing orfuture cellular communications system.

DESCRIPTION OF THE INVENTION

The object of the invention is to provide a system of spread spectrumnavigation beacons arranged geographically in a cellular pattern thatsupports position determination at a mobile or portable cellulartelephone or other wireless communications terminal.

The invention features the following:

1) In one embodiment, the use of a set of direct sequence spreadspectrum signals (with properties described by time slot of operation,specific PN code, PN code phase, and carrier frequency) to comprise acellular array of navigation beacons that is used for position locationby mobile or portable terminals. The system of beacons may be astand-alone system, or an overlay of a cellular communications system inwhich the beacons occupy the same spectrum as the communications system.

2) In another embodiment, the use of a set of chirped spread spectrumsignals to comprise a cellular array of navigation beacons that is usedfor position location by mobile or portable terminals. Again, the systemof beacons may be a stand-alone system, or an overlay of a cellularcommunication system in which the beacons occupy the same spectrum asthe communications system.

3) Navigation beacons that use a common frequency and a common PN code,but are distinguished by a different phase offset of the PN code epochrelative to the 1 msec time epoch. In the terrestrial environment, aunique phase offset in the code relative to the 1 msec epoch can providea unique signature for a navigation beacon in a local geographicalregion.

4) Chirped navigation beacons that use a common frequency, but aredistinguished by a different phase offsets of the chirp epoch relativeto the 1 msec time epoch, and different sweep rates a common frequencyband. In the terrestrial environment, a unique phase offset in the chirprelative to the 1 msec epoch can provide a unique signature for anavigation beacon in a local geographical region composed of a number ofotherwise identical beacons.

5) In a cellular communications system with a cellular positioningsystem (CPS) overlay, the provision of the cellular system controlchannels to convey the navigation "almanac" to mobile and portableusers. The "almanac" is comprised of the data needed to convert a set ofpseudorange measurements into a position, and includes a list of thecellular broadcast locations and a characterization of the navigationbeacons that are broadcast from each location.

6) The use of NVRAM for the storage of the bulk of the "almanac" datawhich is unchanging except insofar as the cellular system and/or itsnavigation beacons are modified as part of system evolution.

7) Direct Sequence Spread Spectrum (DSSS) or Chirped Spread Spectrum(CSS) navigation beacons that are uniquely characterized in a localregion by their assigned signal characteristics so that data modulationof the beacons is not required for beacon identification.

8) Frequency notching in a DSSS or CSS navigation receiver to filter outthe interference caused by the occupied narrow band communicationschannels of a cellular communications system. In the chirped spreadspectrum CSS receiver, the use of signal attenuation when the chirpedspread spectrum CSS signal sweeps through the occupied communicationschannels as a novel implementation of the frequency notch. The use ofthe cellular system broadcast control channels to convey knowledge ofthe occupied slots so that they may be notched from the receiver.

9) The fact that a common antenna and RF front end is applicable forboth communications and navigation is a unique and novel feature of thisinvention.

10) The implementation of the cellular positioning system CPS navigationreceiver/processor using time-domain and frequency-domain approaches.

DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the inventionwill become more apparent when considered with the followingspecification and accompanying drawings wherein:

FIG. 1 illustrates a mobile terminal taking pseudo-range measurements tothree base stations,

FIG. 2 illustrates a hexagonal array of base stations covered by threeorthogonal navigation beacons (NB) represented by different shades ofgray and seven PN codes (or seven phases of a single PN code) which isrepresented by the numbers (1,2,3 . . . 7) in the array,

FIG. 3 illustrates a cellular positioning system of seven navigationbeacons (NBs) that cover a hexagonal region of cells having seven chirpphases of a single chirped signal,

FIG. 4A is a general block diagram of a stand alone base station used inthis invention,

FIG. 4B is a block diagram of a direct sequence spread spectrum (DSSS)embodiment base station used in this invention,

FIG. 4C is a block diagram of a chirped spread spectrum (CSS) embodimentbase station used in this invention,

FIG. 4D is a block diagram of a cellular base station to which thenavigation beacon signals of FIGS. 4A, 4B and 4C has been added,

FIG. 5 is a general block diagram of a cellular position system receiverused in this invention,

FIG. 6 is a block diagram of a cellular position system processor withPN navigation signals using a time domain approach,

FIG. 7 is a block diagram of a cellular position system processor withPN navigation signals using a frequency domain approach,

FIG. 8 is a block diagram of a cellular position system processor withchirp signals, and

FIG. 9 is a block diagram illustrating the time-gated approach forfrequency notching in a chirp receiver.

DETAILED DESCRIPTION OF THE INVENTION

The system is illustrated in FIG. 1 which shows a mobile user MUsurrounded by base stations 10-1, 10-2, 10-3 . . . 10-N of a cellularpositioning/communications system. At any point in time, the mobile unitMU tracks the navigation signals from at least 3 base stations andmeasures the pseudorange to each of them by processing their navigationbeacons. Measurements from three base stations are required for a 2Dsolution and if a 3D solution is desired, then measurements from 4 basestations are required. This positioning system is referred to as theCellular Positioning System (CPS). In the cellular positioning system(CPS), each cellular base station (BS) broadcasts a spread spectrumnavigation beacon (NB) whose frequency and timing are synchronized andslaved to GPS. The system of beacons may be a stand-alone system (FIG.4A), or an overlay of a cellular communication system (FIG. 4D) in whichthe beacons occupy the same spectrum as the communications system. Insuch an overlay system, the power of the spread spectrum signalsdescribed below can be set sufficiently low so as not to interfere withthe communications channels, yet sufficiently high to support a goodsignal for a navigation receiver. Two types of spread spectrumnavigation signals are disclosed herein:

1) A GPS-like direct sequence spread spectrum (DSSS) signal in which thenavigation beacon is a PN coded broadcast (see FIG. 4B). One examplecode has a length of 1023 and a code period of 1 msec so that thechipping rate of the navigation beacon is 1.023 Mcps. The resultingsignal is spread over about 2 MHz of spectrum. Depending upon thedetailed cellular positioning system (CPS) design, the navigation beaconsignal may have a different code length and chipping rate, and maysupport little or no data.

2) A chirped spread spectrum (CSS) signal in which the navigation beaconis a frequency tone that is repeatedly swept over a chosen frequencyband (see FIG. 4C). A 10 MHz band with a period of 1 msec or greater areexample parameters for a chirped signal. Depending upon the chosen CPSdesign, chirp navigation beacon signals may also incorporate differentfrequencies, sweep size and sweep rate, and may support little or nodata.

According to this invention, an efficient set of navigation beacons fora cellular positioning system CPS must conform to a channel structurethat supports orthogonal or near orthogonal beacons that do notsignificantly interfere with each other. The degree of orthogonality isimportant since within a cellular array of navigation beacons, when auser is near a cellular BS, the navigation beacon signal power from thatstation may be 30 dB or more stronger than the power of navigationbeacon signals from far (i.e., adjacent) BSs. With completeorthogonality, the strong navigation beacon will never interfere with aweak navigation beacon. Complete orthogonality is needed to solve thenear-far interference problem in the cellular positioning system CPS.With less than complete orthogonality, however, the near-farinterference is a problem whenever the near signal power is somethreshold number of dB stronger that far navigation beacon signal.

In a cellular positioning system CPS, the same channels can typically bereused for beacons over a wide geographic region because rangeattenuation effectively provides separation between distant beacons thatare using the same channel. Current and proposed cellular communicationssystems use frequency division, time division, code division anddata/tone markers to create distinct channels. The DSSS navigationbeacons in a cellular positioning system CPS similarly can usefrequency, time, code and data to distinguish them from each other. Ingeneral, each individual DSSS navigation beacon is characterized by theparameters in Table 1.

                  TABLE 1                                                         ______________________________________                                        Parameters of DSSS Navigation Beacons                                         ______________________________________                                        time.sub.--slot                                                                      for example, each NB may be on for 1/3 second every second.            of     The time.sub.-- slot would thus be 1, 2 or 3, indicating which         operation:                                                                           part of a 1 second frame the NB is broadcasting in. A scheme                  of 3 time slots supports a 3-fold orthogonality that is complete              and therefore sufficiently robust to solve the near-far problem.              Schemes employing as few as 1, and as many as 7 time slots                    are practical and feasible in a CPS.                                   chosen one of the library of PN codes with good cross correlation             PN code:                                                                             properties. Different PN codes provide a partial orthogonality                that is roughly proportional to the spreading gain (which is                  defined as the receiver signal averaging time divided by the                  chip time duration). Thus for an NB with a 1.023 Mcps                         chipping rate supporting 50 bps of data, the spreading gain                   is about 43 dB which means the signal power of a strong                       signal interferer is suppressed by 43 dB                               phase of                                                                             the phase of the PN code at the msec time epochs. NP signals           the PN may use the same PN codes but broadcast the codes with suf-            code:  ficient phase offset to provide good orthogonality. The code                  phase of the NB may be in-phase (i.e, with the first chip                     aligned to the 1 msec time epoch) or out-of-phase (i.e.,                      delayed by some specified number of chips so that the nth                     chip is aligned to the 1 msec epoch) A 1023 chip code will                    provide roughly 60 dB separation (1023 × 1023) between two              NBs with a different code phase (when averaged over one                       code cycle and the frequencies of the NBs at the receiver are                 coherent over the code cycle period). If the frequencies are                  not coherent, then the orthogonality that is achieved is                      roughly the same as for two different PN codes. A PN code                     with a period of 1 msec provides a substantial amount of                      range ambiguity resolution in the terrestrial environment.                    For example, if a PN code phase is offset 1/2 the code period                 (0.5 msec) with respect to a reference code, the code phase                   can resolve up to 75 km of range ambiguity; thus if the two                   PN codes are broadcast from towers that are closer than 75                    km, each signal can be resolved distinguished from the other.                 The use of more than two code phase states for signals will                   provide proportionately less capability to resolve the                        range ambiguity.                                                       frequency                                                                            Each NB occupies about 2 MHz of spectrum since the band-               of the NB:                                                                           width between the first nulls of an unfiltered 1.023 Mcps                     signal is 2.046 MHz. In a system where multiple signals                       are used for the NBs the nulls of a central frequency are                     good candidates for addition NB frequencies. The 1st null                     results in a completely orthogonal NB as long as the                          frequencies are coherent over the receiver signal integration                 time. Even if the frequencies are not coherent, NBs at the 1st,               2nd, 3rd and other nulls can achieve 30-60 dB or more of                      separation by appropriate filtering.                                   unique ID                                                                            Each NB may support 8-16 bits of data which will be a                  of the NB:                                                                           unique identifier of that NB in the local cellular                            region.                                                                ______________________________________                                    

The chirped spread spectrum CSS navigation becomes in a cellularpositioning system CPS similarly can use frequency, time, sweep phase,sweep rate, and data to distinguish them from each other. Thecharacterization parameters in a system of chirped navigation beaconsare described in Table 2 below. There is much commonalty with the DSSSnavigation beacons but there are some significant differences that arenoted.

                  TABLE 2                                                         ______________________________________                                        Parameters of Chirped Navigation Beacons                                      ______________________________________                                        time.sub.-- slot                                                                     same as for DSSS Navigation Beacons; chirps occupying                  of     different time slots are completely orthogonal.                        operation:                                                                    chirp  Different sweep rates of chirped signals is one form of                sweep  distinct modulation that can provide good orthogonality;               rate:  for example, two chirped signals that occupy the same                         frequency and time slot, but with chirp period of 1 msec                      and 2 msec respectively.                                               phase  the phase of the chirp at the msec time epochs. NB signals             of the may broadcast the same chirp but with sufficient phase                 chirp: offset to provide good orthogonality. The chirp phase of                      the NB may be in-phase (i.e, with the frequency minimum                       aligned to the 1 msec time epoch) or out-of-phase (i.e,                       delayed by some specified amount of time). Chirp signals                      that differ by a constant phase offset are orthogonal.                 frequency                                                                            roughly the same as for the DSSS; chirps at different                  of the NB:                                                                           frequencies are completely orthogonal.                                 unique ID                                                                            Each NB may support 8-16 bits of data which will be a                  of the NB:                                                                           unique identifier of that NB in the local cellular                     ______________________________________                                               region.                                                            

While the above parameters in Tables 1 and 2 apply to the most generalcellular positioning system CPS, in any specific application, the use ofthis parameter set can be limited to a subset that fills out the definedcells with navigation beacons with suitable signal characteristics thatuniquely define them (in a local region) and solve the near-farinterference problem. In order to solve the near-far problem in ahexagonal cellular array, it is necessary to have at least 3 navigationbeacon signals that are virtually completely orthogonal (i.e., separableby at least 60 dB) under all circumstances. Of the set of DSSSparameters listed above, only time-slot and frequency satisfy thisrequirement. While they do not support complete orthogonality, theparameters of PN code and code phase, serve to uniquely identify thenavigation beacon in a local region without having to read a unique IDthat may be modulated onto each navigation beacon. Table 3A illustratesa sample of DSSS navigation beacon signal sets that satisfy theorthogonality requirements to both solve the near-far interferenceproblem and uniquely identify the navigation beacon in its localenvironment. Note that the value define 21 distinct navigation beaconsignals that are suitable for allocation across a hexagonal array ofcells. As illustrated in the cellular positioning system CPS in FIG. 2,the hexagonal array is covered by allocating the 3 orthogonal navigationbeacon signals over the array (represented by the different shades ofgray) and 7 PN codes (or 7 phases of a single PN code) which isrepresented by the numbers in the cellular array. The 3 different shadescan be viewed as either 3 orthogonal time slots or 3 orthogonalfrequencies. In this manner a group of 21 cells is formed and isreplicated repeatedly to fill any cellular array. Note that in thesesystems, the separation distance between navigation beacons with thesame signal parameters is quite large (roughly 9 cell radii).

                  TABLE 3A                                                        ______________________________________                                        Alternative Systems of DSSS Navigation Beacons                                Solve Near-Far Interference                                                                       Provide Unique Signature                                  System time slots                                                                              frequencies                                                                              PN codes                                                                              PN phases                                 ______________________________________                                        A      3         1          1       7                                         B      1         3          1       7                                         C      3         1          7       1                                         D      1         3          7       1                                         ______________________________________                                    

As indicated in Table 3A, many other combinations are possible for thespecification of a set of navigation beacons. Clearly, the variety ofoptions for parameters of time₋₋ slot, frequency, PN code, and PN phaseensure that there are many sets of suitable navigation beacons that canbe generated in a tailored fashion for the particular cellular layoutand constraints on frequency availability and other signal parameters.In an analogous fashion, the parameters of Table 2 that characterize thechirped navigation beacons can be exercised to generate a suitable setof unique beacon signals for any reasonable cellular layout.

                  TABLE 3B                                                        ______________________________________                                        Alternative Systems of CSS Navigation Beacons                                                     Provide Unique                                            Solve Near-Far Interference                                                                       Signature                                                 System time slots                                                                              frequencies                                                                              chirp phases                                                                          chirp rates                               ______________________________________                                        A      3         1          7       1                                         B      1         3          7       1                                         C      1         1          21      1                                         D      1         1          7       1                                         E      1         1          7       2                                         ______________________________________                                    

Table 3B illustrates a sample of CSS navigation beacon signal sets thatsatisfy the orthogonality requirements for solving the near-farinterference problem and for uniquely identifying the beacons in theirlocal environment. For three systems (A-C), 21 distinct navigationbeacons are defined, for one system (D), only seven are defined, and forone (E) fourteen are defined. Note that for chirp signals, chirps ofdifferent phase are orthogonal. Thus different phases alone solve thenear-far interference problem, and so in two systems (C and D), thenavigation beacons are defined only by a distinct phase of the chirp.FIG. 3 illustrates the allocation of 7 distinct chirp phases (as inSystem D of Table 2) over a hexagonal array. The seven phases areassigned to seven hexagons of a local cluster, and this pattern isreplicated to fill an entire coverage area. In general, it can be shownthat in a system of beacons distinguished only by chirp phase thefollowing constraint must be satisfied in choosing the number of phasestates and the period of the chirp for a given cellular coverage area:##EQU1## where D=the diameter of cells

n=the number of distinct phase slates

τ=the period of the chirp, and

c=the speed of light

3.0 Operational Scenarios of CPS Utilization

3.1 A Stand-Alone CPS

A cellular positioning system CPS can be a stand-alone system, or partof a cellular communications system or a combination of stand-alonestations and cellular sites with a positioning signal. In a stand-alonesystem (FIGS. 4A-4C), the position is determined at a navigationreceiver (NR) by acquiring, tracking and demodulating the navigationbeacons. Accordingly, in a stand-alone system, all the navigation datarequired for position determination is broadcast by the navigationbeacons. An example of the data that would be conveyed by eachnavigation beacon is illustrated in Table 4. Note that this informationconveys the position of the BS as well as the signal parameters of thenavigation beacon that is broadcast by that BS.

                  TABLE 4                                                         ______________________________________                                        BS Navigation Beacon Data                                                     ______________________________________                                        Unique BS/Beacon ID                                                           Station Latitude (1 meter quantization)                                       Station Longitude (1 meter quantization)                                      Station Altitude (1 meter quantization)                                       Time.sub.-- Slot and Code.sub.-- Phase and/or other characterization of       NB)                                                                           Selected Parameters of NBs of adjacent BSs                                    ______________________________________                                    

In order to determine position, a navigation receiver NR needs toacquire and measure the pseudorange on at least 3 navigation beacons fora 2D solution (time plus lat/lon), and at least 4 navigation beacons fora 3D solution (time plus lat/lon/alt). In addition, the navigationreceiver NR would also have to demodulate the data on each of thenavigation beacons. When the navigation receiver NR has measured therequired number of pseudoranges and has read the data on each navigationbeacon, the position of the navigation receiver NR can be solved for.Excess measurements can be used to generate added measurement precisionand robustness via standard techniques for minimization of measurementvariance and elimination of out of bounds measurements. The typicalscenario for position determination for a stand-alone cellularpositioning system CPS is as follows:

Step 1: At navigation receiver NR turn-on, the navigation receiver NRsearches the signal parameter space, acquires the first navigationbeacon, and measures the pseudorange. The navigation receiver NR thenreads the data on the navigation beacon.

Step 2: With or without the aid of the data from the first acquirednavigation beacon, the navigation receiver NR continues a search forsubsequent navigation beacons from other BSs. The navigation receiver NRcontinues until it makes a pseudorange measurement and acquires the datafrom at least two additional navigation beacons broadcast from adjacentBSs.

Step 3: The navigation receiver NR computes a 2D or 3D position,depending upon the number of navigation beacons acquired and processed.

Step 4: The navigation receiver NR continues to acquire and tracknavigation beacons as the navigation receiver NR may move though thegeographic region that is covered by the cellular positioning systemCPS, and it periodically recalculates the navigation receiver NRposition according to some defined algorithm.

3.2 A CPS Embedded in a Cellular Telephone System

In a cellular positioning system CPS that is integrated with a cellularcommunications system, the navigation beacons may share the samespectrum as the communications systems (as described in theabove-referenced patent application). In addition, much or all of therequired navigation data may be conveyed by the broadcast controlchannels of the communications system. Thus, in a cellular positioningsystem CPS integrated with a communications system the data supported byeach navigation beacon is minimal: at most an 8-16 bit identifier thatis unique with in the local cellular region. The rest of the navigationdata needed is transmitted by the cellular control channels that arebroadcast from each cellular BS. The scenario for position determinationin this combined communications-navigation system is described below viathe operation of a cellular phone and a phone navigation receiver NR.For illustration purposes, the set of navigation beacon signalsillustrated as System A in Table 3A is assumed.

Step 1. Comm Initialization via Listening to the Cellular ControlChannel: At the start of this scenario, the phone is just turned on. Thephone searches for, finds and listens to a cellular communicationscontrol channel and initializes itself according to the applicablecellular communications standard or protocol.

Step 2. Nav Initialization via Listening to the Cellular ControlChannel: In listening to the control channel, the phone also initializeswith respect to cellular navigation. When this task is complete, thephone has determined that it is near a specified base station, and hasacquired the lat/lon/alt/code₋₋ phase/time₋₋ slot for all the basestations in the local environment. This data is conveyed via navigationoverhead messages that are broadcast on each control channel; theseoverhead messages contain a data set as illustrated in Table 4, but alsoinclude the data for all of the adjacent cellular BSs as well as anissue number that will change with cell system evolution, and thespectral occupancy of communications channel broadcast by the BS. Thisinformation will be stored in non-volatile random access memory (NVRAM)(FIG. 5) so that the data is preserved from call to call. The NVRAM isof sufficient size so the IDs and locations of a reasonable sized regionof cells can be stored. Every time the phone is in an initializationstate, the NVRAM is updated so that the NVRAM always stores the mostcurrent broadcast of the cellular navigation parameters. Thus, in makinga call for which position is desired, the parameters of the localnavigation beacons will be known (assuming that the received "issuenumber" in the navigation data matches the "issue number" of the datastored in NVRAM for that cellular base station). If the received uniqueID and issue number do not both match a value stored in NVRAM, then thenavigation data would need to be collected before the positioning callcould proceed. As the phone explores new territory, the list of cellbase station locations would expand to fill the NVRAM. For a phone thatcovers a great deal of territory, new locations would be written overold locations whenever the capacity of the NVRAM is reached. Thisnavigation data that is stored in the NVRAM is referred to as thenavigation almanac.

Step 3. Search and Acquisition of a Navigation Beacon (NB): This stepcan be done in parallel with Step 1 and Step 2. Every cellular basestation transmits a navigation beacon with a set of signal parametersdesignated in the communication broadcast control channel. Thenavigation receiver NR of the phone searches various time slots and codephase states for a navigation beacon that is strong enough to acquirethe PN code and possibly to read a "unique ID" that is coded onto thenavigation beacon. When this task is completed, the phone navigationreceiver NR has made a pseudorange measurement on the navigation beaconand has reached synchronization with the navigation system, meaning thatit now has an absolute reference for both time₋₋ slot and code₋₋ phase.In the local environment, each navigation beacon is uniquely specifiedby the time₋₋ slot/code₋₋ phase pair so that additional navigationbeacons can be rapidly acquired, and the identity of each navigationbeacon is known a priori (thus the data on these beacons does not needto be read in order to determine the identity of the navigation beacon).

Step 4. Search and Acquisition of Other Navigation Signals:

Having achieved synchronization with code₋₋ phase and time₋₋ slot of anavigation signal in Step 3, the phone navigation receiver NR looks up(in the NVRAM) the code₋₋ phase and time₋₋ slot of other navigationbeacons in the local environment; the phone navigation receiver NR thenproceeds to acquire these signals and make pseudorange measurements.Note that since code₋₋ phase and time₋₋ slot uniquely specify thenavigation beacon in its local environment, the data on these navigationbeacons is not read. Note also that acquisition of navigation beacons inthis Step may be aided by using the data on the spectral occupancy ofcommunications channels via insertion of frequency notches in thenavigation receiver NR to reduce the interference that is created by astrong communications signal (from a nearby cellular base station) on aweak navigation signal (broadcast by a distant base station). Suchnotching would typically be required for the AMPS and TDMA cellularsystems in which the communications channels are contained in 30 KHzfrequency slots that are dispersed throughout the cellular allocation.In a Q-CDMA system, such frequency notching would not be required. Ifpseudorange measurements on at least 2 other navigation beacons aremade, the phone navigation receiver NR can proceed to Step 5.

Step 5. Calculation of Position: With a total of 3 or more pseudorangemeasurements, the navigation receiver NR can generate a 2D solution,solving for navigation receiver NR time and location (lat/lon). With atotal of 4 or more psuedorange measurements, the navigation receiver NRcan generate a 3D solution, solving for time and location (lat/lon/alt).Excess measurements can be used to generate added measurement precisionand robustness via standard techniques for minimization of measurementvariance and elimination of out of bounds measurements. When Step 5 iscompleted, the phone displays a "position fixing" indicator analogous tothe roaming indicator. The "position fixing" indicator will tell theuser that the phone knows its position at that particular moment intime.

Step 6. Recalculation of Position: In the idle state, the phone willcontinue to listen to the communications control channel. During thistime, the phone navigation receiver NR may or may not (e.g., where poweris scarce) continue to operate. In general, the phone navigationreceiver NR will recalculate its position according to a programmedalgorithm. Recalculation could be done continuously, or in response toan expired time or event as described below:

    ______________________________________                                        continuous                                                                             With continuous recalculation, the NR stays on and                   recalculation:                                                                         and continuously recalculates its position. This algorithm                    would be suitable for applications that have                                  sufficient power to support the continuous operation of                       the phone NR.                                                        periodic In this mode, recalculation is initiated periodically after          recalculation:                                                                         the expiration of a set amount of time; thus, the phone                       NR would normally be in a power conserving sleep                              mode, but would wake up periodically (e.g., every 5                           minutes) in order to recalculate its position. The time                       interval between wake-ups would be programmed by the                          phone user.                                                          recalc on                                                                              In this mode, the NR would normally be in a power                    control  conserving sleep mode, but would wake up whenever the                channel  phone receiver changes the control channel that it                   turnover:                                                                              listens to; this turnover in control channels occurs                          whenever the signal strength of the initial control                           channel fades down to a specified level and the phone                         searches for and locks onto a stronger control                       ______________________________________                                                 channel.                                                         

4.0 Operational Scenario of CPS-Cellular Communications

Coordination for Call Processing

At the start of this scenario, it is assumed that the phone and phonenavigation receiver NR has completed the Steps 1 to 3 that are describedabove. At the completion of Step 3, the phone displays a position fixingindicator that tells the phone user that the phone is ready and preparedto make a "position enhanced" (PE) phone call. The call processing thatthen takes place is a follows:

Step 1. Initiating the Call: The phone user initiates a PE phone call inthe same manner as normal calls (nominally by dialing the number andpressing the Send key). Depending upon the way positioning service isused, the phone user may convey a desire for a PE call via the pressingof some specified key combination. For 911 calls, a PE call would be thedefault.

Step 2. Phone Response: In response to the user call initiation, thephone seizes the access control channel and sends a digital message inaccordance with cellular system specifications. This message containsthe phone electronic serial number (ESN), the user mobile phone number(MIN), and other such data. In a PE phone call, the digital messagewould contain an additional cellular control word that conveys thelat/lon of the phone location in accordance with a standard compressedformat. For example, to convey the lat/lon of the phone (with a 10 meterquantization) relative to the lat/lon of the base station would requireabout 24 bits.

Step 3. MTSO Response. With the completion of Step 2, the MTSO has thelocation of the phone prior to call setup, and can therefore use thisinformation in call processing. Thus, in a 911 call, the MTSO could uselocation knowledge to find the appropriate emergency service center forthe phone location, and then route the call and the location data tothat emergency service center. For PE calls other than 911, the MTSOwould also send the location of the calling phone to the calldestination. This can be accomplished in-band via modem (in accordancewith an established standard) or out-of-band via SS7.

Step 4. Call Servicing: Call servicing of a "one-shot" positioning callproceeds the same as a normal cellular call. In response to the requestby the phone, the MTSO assigns an available voice channel to the phonevia a message on the control channel; the phone then switches to thatchannel while the MTSO proceeds to patch the call through to the dialednumber. However, with "continuous" position fixing during a call, thephone navigation receiver NR must continue to measure pseudoranges onall the navigation beacons to update the location estimate of the phone.In order to do this as the phone moves through cell during the callprogress, it is clear that the phone will need to continue to receive acontrol channel in order to maintain the navigation data it needs toacquire and track the navigation beacons. The process for a "continuous"position fixing call is described in the succeeding steps.

Step 5: Call Servicing for "Continuous" Position Fixing: In this mode,the phone navigation receiver NR must continue to receive and monitor acontrol channel, since the ability to position fix depends upon thenavigation overhead data that is broadcast on the control channels.Thus, as phone moves through cells during a call, the phone mustcontinue to monitor control channels and switch to a stronger one asrequired in order to maintain a current file of the navigation data. Asit does this, the phone navigation receiver NR continues to calculateits position. Each position update may then be periodically sent to theMTSO via a specially-defined message on the voice control channel.Alternatively (or in addition), the message can be sent in-band via asimultaneous "data-in-voice" modem. The continuous monitoring of acontrol channel after switching to a voice channel is a departure fromnormal cellular telephony operations, but it is not inconsistent withsuch operations, and may even provide telephony benefits. For example,if the cellular telephone continues to monitor a control channel, theMTSO has a means to offer such services as "call waiting" by alerting aphone via the control channel that a call is being placed to their busynumber.

5.0 Receiver Processing

This section presents a description of selected embodiments of anavigation receiver signal processor. While novel features will beexplicitly identified, it should be emphasized that other embodimentswould also be included as part of this invention. An overview ofreceiver processing, within the user's unit, is shown in FIG. 5. Notethat the user's unit may be a cellular car phone, portable phone, orother receiving device that may, for example, operate in a general PCSenvironment. As seen in FIG. 5, the composite incoming RF signal isreceived by a single antenna (AN) and is amplified and conditioned by asingle RF front end (RFE) (e.g., Low Noise Amplifier). It should beemphasized that the composite incoming signal, which includes thecombination of communication-channel and control-channel signals,typically spans tens of MHz (e.g., >20 MHz for cellular). The fact thata common antenna AN and RF front end RFE is applicable for bothcommunications and navigation is a unique and novel feature of thisinvention.

As further seen in FIG. 5, following the RF front end RFE, the signal issplit SPL into two parts. One path goes to thecommunications/control-channel CPCR processing portion of the device,and the other path goes to the navigation portion NP of the device. Inthe present discussion, only the navigation portion NP of the device isdescribed in detail, but as seen, the navigation processing requires"notching" data that is provided by the Control Channel (CC). This"notching" aspect was addressed and is expanded upon further below.

As further shown in FIG. 5, the signal output of the RF front end RFE isdownconverted via mixer MX, and a single fixed downconversion frequency,to a convenient intermediate frequency (IF). While a single mixing stageis shown, this invention encompasses the m ore general case of one ormultiple downconversion stages. The IF signal is filtered by BandpassFilter BPF, whose bandwidth is approximately 2 Mhz, to accommodate the1.023 Mcps spread spectrum signal or to an alternative bandwidth toaccommodate the chirped signal.

The BPF is followed by the following unique system elements:

1. The Cellular Positioning System CPS Processor, CPSP, which isamenable to a miniaturized/low-power implementation, executes all therequired signal processing functions on the chirped or PNspread-spectrum signals of interest, to enable highly accuratenavigation. The outputs of the CPSP block are the relative PN code orchirp timing epochs of each of the cellular base station BSspread-spectrum signals being tracked; as discussed earlier, up to sevensuch signals may be tracked for a typical hexagonal cellularconfiguration. Also, as discussed in earlier sections, a minimum ofthree "high quality" signals must be tracked to enable a highly accurate2D navigation solution. As such, considerable diversity and robustnessis built into this navigation approach.

2. The timing and other data output from the CPSP is fed into theNavigation Processor block NP, which converts the relative timinginformation from CPSP into actual user position (e.g.,latitude/longitude).

While several embodiments of the CPSP block may exist, two uniqueembodiments for DSSS navigation beacons are illustrated in FIGS. 6 and7, respectively. In addition, one embodiment of the CPSP block for CSSnavigation beacons is illustrated in FIG. 8. It should be emphasized,however, that other embodiments that may exist would also be included inthis invention. Key features of each of the processing approaches aredescribed in the following.

CPs Processor 1 (for DSSS Navigation Beacons)--Time Domain Approachusing CCD Correlators

The heart of the CPSP in FIG. 6 is the serial combination of the twoCharge Coupled Device (CCD) blocks shown. The processing shown here is aunique application and extension of the IF sampled CCD demodulationdescribed in the above-referenced patent and another patent applicationthat is currently pending. The key features of this processing are asfollows:

1. The IF is sampled at a carefully selected rate, so that successivesamples effectively represent in-phase and quadrature basebandcomponents. This sampling rate is (4/k)×the IF, where k is an oddinteger. In other words, baseband components are generated here withoutthe need for mixer components. Also, these operations are performed athigh speed and low power consumption without A/D conversion, since theCCD is an analog device. As seen in FIG. 6, a single A/D converter isrequired after all CCD processing is complete. Further details aredescribed in the above cited patent and patent application, which alsodiscusses the significant benefits in size, power consumption, andprogrammable flexibility.

2. The first CCD is implemented as a programmable transversal filter,CTF, whose tap weights may be programmed with multi-bit (e.g., 8)quantization, to enable shaped filtering. In the present application,the specific CTF of interest "notches" out the high-power, narrowbandcommunication signals that may degrade spread-spectrum signaldemodulation.

a. When the user's unit is first turned on, it tunes to the cellularControl Channel (CC) designated to the BS for the cell that the user isin. For AMPS and TDMA, this represents a 30 KHz channel, while forQCDMA, the control channel is another spread-spectrum signal. For AMPSand TDMA, immediately following turn-on, the CTF shown isbypassed--i.e., the switch shown is closed--thereby precluding notching.The reason for this is that notching is not required to acquire andaccurately track the spread spectrum signal transmitted by the BS thatis closest to the user; this has been explicitly shown in our previouspatent application (Position Enhanced Communication System IncludingSystem for Embedding CDMA Navigation Beacons Under the CommunicationsSignals of a Wireless Communication System)). The CCD following thetransversal filter is used to acquire and track the desiredspread-spectrum signal this CCD is discussed further below.

b. The CTF is always bypassed for QCDMA, since notching is never neededwhen operating in the QCDMA spread-spectrum environment.

c. The AMPS or TDMA CC provides the CPSP with the frequencies of thecommunication channels employed by the BS in the user's cell. once thisdata is received by the CPSP, it programs the taps of the CTF to "notch"out these "strong" narrowband signals, thereby virtually eliminating the"near-far" problem. This notching remains in place as navigationprocessing is in progress. Furthermore, when the user moves to anothercell, and the frequencies of the "strong" communication channels change,the CTF tap weights are appropriately adjusted based on data provided bythe new CC designated to the user's new cell BS.

3. The CTF is followed by another CCD, CCF, that is sampled at the samerate as the CTF. CCD, CCF has fixed tap weights matched to the PN codeof interest, and provides the extremely rapid PN code acquisition thatis essential for applications such as 911. The uniqueness of thisimplementation is noted. Specifically, for the Table 3A System A ofinterest, a single CCD PN matched filter correlator is all that isneeded to process all CPS spread spectrum signals across the entirecellular system. This is because the same PN code is transmitted by allcell BSs, with discrimination among sites being executed via thecombination of the time diversity and code phase diversity described inSections 3 and 4.

4. The CCD, CCF output is A/D converted, to enable efficientpost-processing as shown. The A/D operation is advantageous here sincethe CCD output SNR is much higher than at the input. Furthermore, CCDtechnology has advanced to the point where an ultra-low power AID may bedirectly incorporated onto th e CCD chip itself, wherein the A/D isimplemented using "charge-domain" processing techniques.

5. The A/D output is averaged and algorithmically processed to determinethe PN code correlation peaks--hence, their relative timings--therebyyielding the desired timing data for transfer to the NavigationProcessor. As also noted in FIG. 6, the algorithmic processing of theDigital Processor, DP, further provides AFC frequency controlcorrections, as necessary, to compensate for offsets in the user's localoscillator. These frequency corrections are executed by suitableadjustment of the CCD IF sampling rate--another unique feature of thisimplementation!

CPS Processor 2 (for DSSS Navigation Beacons)--Frequency Domain Approach

A frequency domain equivalent to the above is shown in FIG. 7. In thisscenario, the IF input to the CPSP is first A/D converted, and allsubsequent processing is performed digitally:

1. The procedure at service turn-on is virtually identical to the timedomain approach, in that the CC of the user's BS is first processed toidentify the spectral locations of the "strong" communication channelsassociated with the user's BS.

2. For this FFT process fixed blocks of data are collected and stored ata time. In the present illustrative case, wherein the IF is sampled atfour times the PN chip rate, each block of 4×1023 samples encompassesthe full PN code cycle. The collected samples are then used to generatethe associated FFT (zero padding may also be used, as necessary ordesired for additional resolution).

3. For initial acquisition and tracking of the user BS's cellularpositioning system CPS signal, no notching is required, as discussed inthe time domain approach. Once locations of the "strong" signals areidentified via the CC, notching is readily implemented by zeroing outthe appropriate portions of the FFT.

4. PN code correlation/despreading is implemented in the frequencydomain by multiplying the FFT by the complex conjugate of the FFT of thecellular positioning system CPS PN code; this complex conjugate isstored as the array H*(w).

5. The result of the multiplication is processed by an inverse FFT(IFFT) to yield the time-domain correlation function. Once this IFFT isaveraged, to enhance SNR, the desired correlation peak--and, hence, thedesired PN code epoch--is readily extracted. The IFFT may further beprocessed to yield frequency correction information, to correct forlocal oscillator offset, and the result fed back to the Complex Multiplyblock shown, CM.

6. As in the time-domain approach, for System A of Table 1, three PNcode epochs are obtained from each IFFT, wherein the epochs are suitablyand unambiguously spaced.

7. Because the FFT and IFFT operations are computationallyintensive--especially when the length is on the order of 4000 orgreater--the FFT/IFFT operations need not be performed on contiguousdata. Thus, for example, a 1 ms data block (1 PN code cycle) iscollected and processed over several ms, followed by additional datacollection and processing. Because each FFT/IFFT encompasses three PNcode epochs, this mode of operations still offers efficient acquisitionand tracking at acceptable levels of computation power.

CPS Processor (for CSS Navigation Beacon)

A comparable processing chain for CSS beacons is illustrated in FIG. 8.The CSS beacons are assumed to have a period of 5 msec during which theysweep over 10 MHz of frequency. In this implementation, the key featuresof the signal processing are as follows:

1. The signals enter at RF and are mixed with the output of asynthesizer SYN. The synthesizer SYN produces a frequency staircase at10 KHz steps (each lasting 5 μsec). Thus, in 5 msec (1000 stairs), thesynthesizer moves over 10 MHz of frequency. The nominal intermediatefrequency (IF) resulting from the mixing process is 70 MHz. In general,there will be a number of chirped navigation signals arriving at thenavigation receiver, along with a number of narrowband signals of thecommunications system. At the outset, the navigation receiver conducts asearch to first acquire a strong navigation signal. The process by whichthis is done is a "largest of" detection algorithm based upon a searchover all phases of chirp. This process of acquisition and tracking ofthe first signal is described in items 2 and 3 below. The acquisitionand tracking of subsequent navigation beacons is described in item 4below.

2. During acquisition, the output of the mixing process due to areceived chirped navigation beacon is a series of short chirps each witha duration of 5 usec. If the synthesizer is roughly in phase with thenavigation signal, the chirps will be within 10 KHz of the 70 MHz IF. Ifthe synthesizer is out of phase with the incoming chirped signal, thechirps after the mixing process can be up to ± MHz offset from the 70MHz IF. The CCD sampling rate is 20 MHz and the CCD has fixed binary tapweights so that alternate output samples of the CCD are a moving sum ofthe in-phase (I) and quadrature (Q) components of the input signal. Themoving sum over 5 μsec is essentially a low-pass filter with a firstnull out at 200 KHz. The output of the CCD is then sampled by the A/Dstage of the receiver at 5 μsec intervals (after each frequency stair).Thus the sampling rate of the A/D for I/Q pairs is 200 KHz, and furtherprocessing by the navigation receiver is all digital.

a. Following the A/D step, the signal is further accumulated coherentlyfor 0.1 msec which is 20 samples of the CCD; with this step, the filterbandwidth of the receiver has been collapsed down to 10 KHz to the firstnull. The maximum allowable time for coherent accumulation is controlledby the drift in the local oscillator (that is driving the synthesizer)relative to the incoming received navigation signal. If the localoscillator (LO) is accurate to ±1 part in a million, the relativefrequency drift for a 900 MHz navigation signal will be about ±900 Hz.Over 0.1 msec, a 900 Hz offset will result in about a 30° drift in thephase of the I and Q samples. For drifts larger than this, cancellationamong the accumulated samples will occur.

b. This coherent accumulation is followed by envelope accumulation for 5msec (50 coherent accumulation intervals); this value is then dumped andstored in memory. This value stores the detection statistic for a searchof an incoming chirp signal with a bandwidth that is roughly within 5KHz of the chirp that is generated by the synthesizer.

c. At this point, the timing of the frequency staircase is advanced by2.5 μsec and the accumulation resumes again for 5 msec; the resultingoutput is again dumped to memory. The effect of the 2.5 μsec advance inthe phase of the synthesizer is a shift in the frequency searchbandwidth of 5 KHz.

d. This process continues for 2000 iterations so that all phases of thecode are explored with a quantization of 2.5 μsec in time and 5 KHz infrequency (e.g., 2000×5 KHz=10 MHz); the largest of the 200 iterationsis chosen, and with this detection, the phase of the strongest chirpednavigation signal has been acquired. The estimated acquisition time forthis first signal is thus 2000×5 msec or about 10 seconds.

3. During tracking, the control of the synthesizer is turned over to atracking loop. As in acquisition, the CCD output is a series of I/Qpairs at a 200 KHz rate . The DSP then implements an accumulator over atime interval of from 1 msec to 10 msec which corresponds to a trackingbandwidth of 1000 Hz and 100 Hz, respectively. The tracking accurate(Δx) is dependent upon the resolution of the frequency trackingbandwidth and is governed by the following formula: ##EQU2## where,Δƒ=frequency resolution

τ=the period of the chirp=5 msec

C=the speed of light, and

F=chirp swing=10 MHz

Thus, a 100 Hz resolution corresponds to a 15 meter accuracy for thepseudorange tracking measurement. However, multipath delays of up 1 μsec(300 meters) can be expected; thus, at times this will create abroadening of the frequency on the order of 1000 Hz. Thus, it isnecessary to have an adaptive tracking loop that spans the frequencyband created by the multipath.

4. Upon acquisition and tracking of a first navigation signal, theparallel receiving channel in FIG. 8 begins operation. The acquisitionand tracking for subsequent navigation signals is similar to that of thefirst navigation signal, but there are some significant differences.Firstly, with the acquisition of a first navigation signal, the searchspace for the chirp phase is reduced by about a factor of 10 so thatsubsequent acquisitions are accomplished in about 1 second. In addition,there is a reasonable chance that the subsequent navigation signals willbe interfered with by strong narrowband communications signals from anearby cell site. In this case, this interference must be mitigated by atime-gated switch that eliminates this interference by nulling out thereceived signal when the synthesizer passes through the occupiednarrowband communications channels. In this manner, the communicationschannels can be attenuated by 40 dB or more, and allow the reception ofmuch weaker navigation signals from more distant cell site. To achievethis nulling, a sliding window of FIR filter is implemented that has asharp roll-off so that interfering signals that fall outside the bandare greatly attenuated; when the interfering signals cross the FIRbandwidth, the time gate nulls out the incoming signal. Thus the FIRfilter, in combination with nulling at the appropriate times,effectively and dramatically reduces the interference resulting fromstrong communications signals during both acquisition and tracking. Thisconcept is illustrated in the functional receiver design shown in FIG.9. The wideband chirped signal and the narrowband interferer fromwideband filter WF are correlated in correlator COR with signals fromchirp generator CG (with frequency offset) and under control ofaccumulator/detector ACD. An electronic switch or time gate ES is openedwhen the interferer crosses the passband frequency. The correlation peakof the incoming chirp and the mooring peak of the interfering signalsare shown in the inset in the lower right side of FIG. 9.

SUMMARIZING THE INVENTION HAS THE FOLLOWING NOVEL FEATURES

The use of a set of direct sequence spread spectrum signals (withproperties described by time slot of operation, specific PN code, PNcode phase, and carrier frequency) to comprise a cellular array ofnavigation beacons that is used for position location by mobile orportable terminals. The system of beacons may be a stand-alone system,or an overlay of a cellular communications system in which the beaconsoccupy the same spectrum as the communications system.

The use of a set of chirped spread spectrum signals to comprise acellular array of navigation beacons that is used for position locationby mobile or portable terminals. The system of beacons may be astand-alone system, or an overlay of a cellular communication system inwhich the beacons occupy the same spectrum as the communications system.

The use of navigation beacons that use a common frequency and a commonPN code, but are distinguished by a different phase offset of the PNcode epoch relative to the 1 msec time epoch. In the terrestrialenvironment, a unique phase offset in the code relative to the 1 msectime epoch can provide a unique signature for a navigation beacon in alocal geographical region.

The use of chirped navigation beacons that use a common frequency, butare distinguished by different phase offsets of the chirp relative tothe 1 msec time epoch, and different sweep rates within a commonfrequency band. In the terrestrial environment, a unique phase offset inthe chirp relative to the 1 msec time epoch can provide a uniquesignature for a navigation beacon in a local geographical regioncomposed of a number of otherwise identical beacons.

In a cellular communications system with a cellular positioning systemCPS overlay, the use of the cellular system control channels to conveythe navigation "almanac" to mobile and portable users. The "almanac" iscomprised of the data needed to convert a set of pseudorangemeasurements into a position, and includes a list of the cellularbroadcast locations and a characterization of the navigation beaconsthat are broadcast from each location.

The use of NVRAM for the storage of the bulk of the "almanac" data whichis unchanging except insofar as the cellular system and/or itsnavigation beacons are modified as part of system evolution.

The use of DSSS or chirped spread spectrum CSS navigation beacons thatare uniquely characterized in a local region by their assigned signalcharacteristics so that data modulation of the beacons is not requiredfor beacon identification.

The use of frequency notching in a DSSS or chirped spread spectrum CSSnavigation receiver to filter out the interference caused by theoccupied narrow band communications channels of a cellularcommunications system. In the chirped spread spectrum CSS receiver, theuse of signal attenuation when the chirped spread spectrum CSS signalsweeps through the occupied communications channels as a novelimplementation of the frequency notch. The use of a cellular systembroadcast control channels to convey knowledge of the occupied slotsthat they maybe notched from the receiver.

The fact that a common antenna and RF front end is applicable for bothcommunications and navigation is a unique and novel feature of thisinvention.

The implementation of the cellular positioning system CPS navigationreceiver/processor using time-domain and frequency-domain approaches.

While the invention and preferred embodiments have been shown anddescribed, it will be appreciated that various other embodiments,modifications and adaptations of the invention will be readily apparentto those skilled in the art.

What is claimed is:
 1. Positioning system for locating a mobile body comprising a plurality of earth based spread spectrum (SS) broadcasting stations arranged geographically in a hexagonal cellular pattern, each said SS broadcasting station including a modulator providing a channel signal structure which is substantially orthogonal with respect to adjacent SS broadcasting stations in said hexagonal cellular pattern, each channel signal including navigation beacon data including:(a) a unique beacon identification, (b) station latitude and longitude, (c) time slot frequency and phase characterizations and (d) selected parameters of adjacent stations.
 2. The positioning system defined in claim 1 wherein each said modulator provides a GPS like direct sequence SS signal in which the navigation beacon data is a PN coded broadcast.
 3. The positioning system defined in claim 1 including receiver means for receiving the SS signals from at least three of said SS broadcasting stations and determining the location thereof.
 4. The positioning system defined in claim 1 in which GPS satellite signals are received in said hexagonal cellular pattern and including means at each said broadcasting station for receiving said GPS satellite signals and deriving therefrom a reference frequency signal and a time epoch signal, said modulator means being adapted to receive said reference frequency and time epoch signals and be timed thereby.
 5. Positioning system for locating a mobile body comprising a plurality of earth based spread spectrum (SS) broadcasting stations arranged geographically in a hexagonal cellular pattern, each said SS broadcasting station including a modulator providing a channel signal structure which is substantially orthogonal with respect to adjacent SS broadcasting stations in said hexagonal cellular pattern, each channel signal including navigation beacon data including:(a) a unique beacon identification, (b) station latitude and longitude, (c) time slot frequency and phase characterizations and (d) selected parameters of adjacent stations, each said modulator providing a GPS like direct sequence SS signal in which the navigation beacon is a PN coded broadcast, and receiver means for receiving the direct sequence SS signals from at least three of said SS broadcasting stations and determining the location thereof.
 6. The positioning system defined in claim 5 in which each said SS broadcasting station includes means to receive GPS satellite signals and means for deriving therefrom a reference frequency signal and a time epoch signal, said modulator means being adapted to receive said reference frequency and time epoch signals and be timed thereby. 